The present invention relates to a method for selecting a semiconductor laser and, more particularly, to a method for selecting a self-pulsation type semiconductor laser which is utilized as a basic device in optoelectronics and suitably applied in particular to the fields of optical recording, optical communications and the like. The present invention plays an important role in obtaining a semiconductor laser having excellent reliability which can be manufactured with improved productivity.
In various technical fields utilizing semiconductor lasers, there have often arisen problems of noise caused by the coherence which laser radiation generated by such semiconductor lasers has. In the field of optical communications in particular, there exists a problem of noise (modal noise) produced by constructive and destructive interference among modes at the output end of an optical fiber when laser radiation generated by a semiconductor laser is made to propagate through a multimode optical fiber (refer, for example, to Publication of Japanese Electronics and Data Communications Society, Jan. 1989, pp. 60, "Application of Laser Diode for Public Use to Optical Communications" by Yoneda and Shudoh).
As a means for avoiding such a problem, it has been attempted, by using a semiconductor laser that exhibits a high frequency self-pulsation even under a continuous steady state operation, to eliminate the interference among modes.
There have hitherto been known such a self-pulsation semiconductor lasers of: the type (i) wherein a saturable absorbing layer is formed in a cladding layer (as disclosed in, for example, Japanese Unexamined Patent Publications Nos. 171186/1986 and 202083/1988); the type (ii) wherein the width of a stripe and the thickness of a cladding layer are optimized (as disclosed in, for example, Japanese Unexamined Patent Publication No. 101089/1986); the type (iii) wherein an active layer is of a quantum well structure (as disclosed in, for example, Japanese Unexamined Patent Publications Nos. 72688/1990 and 78290/1990); the type (iv) wherein electrodes for current injection are separated from each other (as disclosed in, for example, Japanese Unexamined Patent Publication No. 5680/1987); and the type (v) wherein each laser employs an original structure for suppressing carrier diffusion in the lateral direction in terms of cross section (as disclosed in, for example, Japanese Unexamined Patent Publications Nos. 133789/1987, 97389/1987, 97385/1987 and 86783/1987).
Nevertheless, semiconductor lasers of the type (i) involved a problem of poor reliability due to generation of heat in the saturable absorbing layer. With the type (ii), the width of a stripe and the thickness of a cladding layer needed to be precisely controlled, thereby cause a problem of low productivity. As far as the type (iii) of the semiconductor lasers were of the structure having a stripe of a single width, the width or number of wells needed to be precisely controlled for forming a quantum well structure capable of generating favorable self-pulsation, and further the rate of crystal growth needed to be made low for formation of a good quantum well, thereby cause a problem of low productivity as with the type (ii). Since the type (iv) of the semiconductor lasers were of a separated electrode structure, there was to add another microfabrication step, i.e. second resist process in the production process, thereby cause a problem of low productivity. Further, with various semiconductor lasers proposed as the type (v), they were each of a complicated structure which required a complicated production process, thus lowering the productivity thereof.
Further, all the semiconductor lasers thus fabricated did not necessarily exhibit self-pulsation and, hence, lasers not exhibiting self-pulsation could be present among products, depending upon the production process. In selecting semiconductor lasers exhibiting self-pulsation by distinguishing from those not exhibiting self-pulsation, the pulsation waveform of each laser was displayed by oscilloscope or the like so as to be visually judged whether the laser exhibits self-pulsation or not. Since the visual judgment was difficult and could not be automated, semiconductor lasers were produced with decreased productivity and reliability.